Variability in muscle fibre areas in whole human quadriceps muscle: effects of increasing age.

Cross-sections of whole vastus lateralis muscle from 20 men, 19 to 84 years of age, were prepared, and the cross-sectional area (microns2) of 375 type 1 and 375 Type 2 fibres was measured in five different regions throughout each muscle. In muscles from the old individuals, the mean CSA of Type 2 fibres was on average nearly 35% smaller (P less than 0.001) while the mean CSA of Type 1 fibres was on average just over 6% smaller (NS) than in muscles from the young individuals. There was a highly significant (P less than 0.001) variation in the mean CSA of both fibre types within all muscles. In the old muscles, there was no significant difference in mean fibre CSA between deep and superficial parts while in the young muscles the mean fibre CSA was significantly (P less than 0.05) larger in deep regions than superficially. The range of the fibre CSA was larger in the old muscles with an increased number of both hypotrophied and atrophied fibres as well as large, sometimes very large, fibres. The standard deviation of the fibre CSA of Type 2 fibres was significantly (P less than 0.001) larger than for Type 1 fibres in 60% of the regions of the old muscles compared to 12.5% of the regions of the young muscles, but the standard deviation for the whole muscles was more or less unaffected with increasing age. In the old age group, there were fewer muscles and regions with a correlation between the CSA of Type 1 and Type 2 fibres than in the young age group. In conclusion the age-related changes in the mean fibre CSA, and in the pattern of variation in fibre CSA throughout the muscle and in small sample regions, suggest a combination of a progressive denervation process and an altered physical activity level as the two major mechanisms underlying the effects of normal development and ageing on the human vastus lateralis muscle.